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Overview
UWB (Ultra Wide Band) is a carrier-free communication technology that uses nanosecond non-sinusoidal narrow pulses to transmit data, so it occupies a wide spectrum range. Traditional positioning technology determines the position of an object based on the strength of the signal. The strength of the signal is greatly affected by the outside world, so the error between the located object position and the actual position is also large, and the positioning accuracy is not high. UWB positioning uses broadband pulse communication technology, which has strong anti-interference ability and reduces positioning errors. The emergence of UWB positioning technology fills the gap in the field of high-precision positioning. It has the advantages of being insensitive to channel fading, low power spectrum density of transmitted signals, low interception capability, low system complexity, and centimeter-level positioning accuracy.
UWB Indoor Positioning Process
(1) Each positioning tag continuously sends data frames using UWB pulses.
(2) The UWB pulse string sent by the positioning tag is received by the positioning base station;
(3) Each positioning base station uses a high-sensitivity short pulse detector to measure the time it takes for each positioning tag’s data frame to arrive at the receiver antenna;
(4) The positioning engine refers to the calibration data sent by the tag to determine the time difference between the tag reaching different positioning base stations, and uses three-point positioning technology and optimization algorithms to calculate the tag position.
(5) When using a single base station for positioning, the AOA (Angle of Arrival) algorithm is generally used, while when using multiple base stations for positioning, the TDOA (Time difference of Arrival) algorithm is often used.
UWB indoor positioning system architecture and functions
System structure diagram
Main equipment and components
UWB positioning tag
The positioning tag is an active tag that can be made into different forms and fixed on objects, vehicles or worn on people. It has variability in different application environments. Its positioning accuracy can reach up to 5-10cm. The UWB pulse signal emitted by the tag is received and transmitted by the positioning base station (positioning sensor). Each tag has a unique ID number, which can be used to link the located object, so that the positioning base station (positioning sensor) can find the actual location through the tag. The tag transmission signal lasts for a very short time, which can allow hundreds of tags to be located at the same time.
Provide a reference technical parameter:
UWB positioning base station
The positioning base station (positioning sensor) can determine the location of the tag through the time difference of arrival (TDOA) measurement technology, and transmit the data to the network controller and positioning engine software, with positioning accuracy reaching centimeter level.
Positioning Engine
The positioning engine is deployed in the positioning server, where the positioning algorithm is implemented and provides API and upper-layer application software calls.
Upper layer application software
The upper layer application software is mainly reflected in the system functions. The basic functions of the UWB indoor positioning system may include:
Personnel distribution display: distribution of personnel wearing positioning tags.
Personnel movement trajectory: Display the movement trajectory of one or more personnel in real time through an electronic map.
Track a specific person: The map automatically switches as the label moves. Move the mouse over the label to see the current status of the label.
Electronic fence: Displays the alarm location status of personnel through different colors and shapes on a flat electronic map.
UWB Technology Features
UWB technology is applied to indoor positioning and has the following characteristics:
(1) Large system capacity
Shannon's formula gives C=Blog2(1+S/N). It can be seen that the increase in bandwidth makes the channel capacity increase far greater than the effect of the increase in signal power. This is also the theoretical mechanism for the proposal of ultra-wideband technology. The number of users of ultra-wideband radio systems is much higher than that of 3G systems.
(2) Fast data transmission speed
UWB system uses ultra-wide frequency band of gigahertz. According to Shannon's channel capacity formula, even if the power density of the transmitted signal is controlled very low, a high information rate can be achieved. Generally speaking, its maximum data transmission speed can reach hundreds of megabits per second to gigabits per second.
(3) Strong multipath resolution capability
UWB has a very high resolution due to its extremely high operating frequency and extremely low duty cycle. The multipath signals of narrow pulses are not easy to overlap in time, and it is easy to separate the multipath components, so the energy of the transmitted signal can be fully utilized. Experiments show that in a multipath environment where the multipath fading of conventional radio signals is as deep as 10 to 30 dB, the fading of UWB signals is less than 5 dB at most.
(4) Good concealment
Because UWB has a very wide spectrum and very low energy density, information transmission is highly secure. On the other hand, due to its low energy density, UWB devices have very low interference with other devices.
(5) Accurate positioning
Impulse pulses have high positioning accuracy. Using ultra-wideband radio communication, they can accurately locate indoors and underground, with an accuracy of up to 2 cm and a general accuracy of within 15 cm. The GPS positioning system can only work within the visible range of the GPS positioning satellite. Unlike GPS, which provides absolute geographic location, ultrashort pulse locators can give relative locations with a positioning accuracy of up to centimeters.
(6) Strong anti-interference ability
The processing gain of UWB spread spectrum depends mainly on the duty cycle of the pulse and the number of pulses used to send each bit. The duty cycle of UWB is generally 0.01 to 0.001, which has a much higher processing gain than other spread spectrum systems and strong anti-interference ability. Generally speaking, the anti-interference processing gain of UWB is above 50dB.
(7) Low power consumption
The receiver of the UWB wireless communication system does not have a local oscillator, power amplifier, phase-locked loop (PLL), voltage-controlled oscillator (VCO), mixer, etc., so the structure is simple and the equipment cost will be very low. Since the UWB signal does not require a carrier, but uses intermittent pulses to send data, the pulse duration is very short, generally between 0.20 and 1.5ns, and has a very low duty cycle, so it only requires a very low power supply. Generally, the UWB system only needs 50 to 70mW of power, which is one-tenth of Bluetooth technology.
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